The capability of modulating the intensity of light in a fiber at high frequencies, e.g. RF or microwave frequencies, is essential to the development of a variety of advanced electromagnetic sensors, to the analog transmission of information over fiber, and to optical signal processing. A variety of techniques have been developed, the most successful being the integrated optic Mach-Zehnder interferometer implemented on lithium niobate. The Mach-Zehnder interferometer is an optical device wherein input light is split and travels along two continuous paths, and is interfered. The two optical paths may be of different lengths, so that the two beams may interfere constructively or destructively. Lithium niobate is an electro-optic material, such that its index of refraction, and thus the optical path length traveled by light passing through the material, may be varied by the application of an electric field. In this manner, the Mach-Zehnder interferometer with a lithium niobate modulator effectively acts as an amplitude modulator of the optical signal propagating in the interferometer.
Despite the broad application of Mach-Zehnder amplitude modulators, they have a number of drawbacks, chief among which is bias drift. Typically systems would require the modulator to operate at particular point in its transmission characteristic, i.e. at a particular bias. For example, for linear voltage response as is required in analog systems (such as cable television distribution networks, or radar systems), the Mach-Zehnder modulator is operated at the 90.degree. (quadrature) phase bias. Improper bias causes undesirable effects, such as an increase in the harmonic distortion of the transmitted optical signal. In general, it is impossible to fabricate a modulator with the proper intrinsic bias. Thus the bias is usually set by application of a DC voltage. However, the required bias voltage may vary over time due to external environmental factors (e.g. temperature, acoustic effects), or internal factors (intrinsic field screening by long-term charge transport in the modulator's layers). Both effects can easily swing the bias phase over a full 2.pi. radians (360.degree.) on short time scales, so that some means of complicated feedback controlled bias tracking is essential to extend the usable lifetime of a modulator in practical application. This has spurred considerable research-mostly unsuccessful-towards the development of an intrinsically more stable electro-optic modulator. Thus other approaches which can address these problems would be most welcome.
One such approach employs a Sagnac interferometer, similarly biased at quadrature, with an electro-optic phase modulator disposed in the interferometer's loop. The Sagnac interferometer operates by counterpropagating signals in the same optical path before interfering them. Thus any slow drift in material parameters (i.e. much slower than the period of propagation though the interferometric loop) will cancel. Recently, relatively broadband and linear operation of a Sagnac interferometric amplitude modulator (or "SIAM") has been demonstrated at modulating frequencies in excess of 500 MHz. See, U.S. patent application Ser. No. 08/690,035, U.S. Pat. No. 5,596,171, filed Jul. 31, 1996, and currently pending; the substance of this application is incorporated herein by reference. Unfortunately, the frequency response of this configuration is not flat. It is thus of interest to extend the effective operating range of Sagnac based modulators to lower frequencies, and to flatten the frequency response of such modulators.